Perform a site search on: Yanche's Toolbox, Piping & Circulator Selection Method. Yanche supplies all the information you'll need to calculate your problem. It's all about the pump supply curve versus the load curve; and, finding the point where the two curves are equal. Bearing in mind: you don't really need to know the exact point where they are equal! You only really need to best approximate your load curve; and, ensure your selected pump can supply enough pressure to get the gallon per minute flow rate required to transfer the BTUs you need.
As an example: I'm heating my in-ground pool with my AHS260 via a two stage system: 1" copper from the boiler to the end of my basement, transitioning to 1" pex via a plate exchanger. I have Taco #13 pumps on both the primary loop directly off the S260 boiler; and, the secondary loop off the plate exchanger. The plate exchanger then feeds the heat exchanger at the pool filter assembly. I'm able to transfer all the heat the S260 can supply, to the pool, very effectively with the Taco #13 pumps; even though, I'm only using 1" copper and pex.
When plumbing my S260 to the pool, I really wanted to use 1.25" copper and pex; but, quickly discovered 1.25" copper was outrageously expensive and 1.25" pex wasn't readily available for some odd reason. You'd think the outdoor wood boiler folks would be using 1.25" pex for heat transfer... Go figure...
Taco #13 pumps can really move a lot of water. I found them on-line for about $160. The local plumbing supply shop wanted $350!!
Good luck with your plumbing endeavour.